Selective catalytic reduction (SCR) is a technology for reduction of nitrogen oxide (NOx) emissions from combined and simple cycle power plants and coal-fired boilers. The process involves injecting NH3 into the exhaust gas upstream of a catalytic reactor. Within the catalyst bed, NOx reacts with adsorbed NH3 in the presence of O2 to form primarily N2 and water. Traditional catalysts employed by the process consist of V2O5 supported on high-surface-area TiO2. The process achieves up to 90-95% NOx reduction efficiency at low to moderate reaction temperatures (300-420° C.). Traditional SCR processes are operated with stoichiometric NH3/NOx ratios consistent with an 80-90% NOx reduction. Operation with higher NH3/NOx ratios enhances the NOx reduction efficiency at the expense of ammonia slip, which often needs to be below than 2-5 ppm. NH3 slip contributes to emissions of nitrogen compound into the atmosphere and leads to the formation of corrosive ammonium sulfates and bisulfates downstream of the SCR. These compounds also may plug heat exchange surfaces in the heat recovery steam generator (HRSG).
For combined-cycle power plants, the SCR reactor is integrated into the HRSG. In this manner, the catalyst temperature under normal operation conditions of 50-100% load is between 300 and 370° C. However, in simple cycle power plants the temperature of the exhaust leaving a high efficiency gas turbine can exceed 600° C. Absent any heat exchange surfaces in the post turbine enclosure, the reactions between ammonia and NOx encounter these high temperatures. This requires an SCR catalyst durable to high temperatures and thermal shock, since the temperature in the gas turbine exhaust reaches its maximum in a short period of time, such as 10 minutes.
High temperature of the exhaust gases requires a reduction in the distance between NH3 injection plane and a face of SCR in order to minimize substantial decomposition of ammonia at temperatures above 500° C. A short distance between ammonia injection grid and the face of SCR dictates the installation of special distribution/mixing devices in order to mitigate maldistribution problems resulting from shrinking zone for ammonia and NOx upstream mixing.
Locating a V2O5/TiO2 SCR catalyst downstream of a gas turbine in temperatures above 550° C. is not feasible, as the temperatures of the flue gas will rapidly deactivate the V2O5/TiO2 catalyst, due to a transformation of the TiO2 from the high surface area anatase phase to the low surface area rutile phase.
Acidified, metal-impregnated, zeolite-based SCR catalysts have been investigated as an alternative to the V2O5/TiO2 SCR catalyst (Farnos et al. 1996; Parvulescu et al. 1998; Stevenson and Vartuli, 2002; Qi and Yang, 2004). Zeolite based catalysts are being investigated because they offer advantages over V2O5/TiO2 SCR catalysts, namely wider operating temperature range, greater thermal stability, and reduced disposal issues associated with the spent catalyst. Although many types of zeolites have been found to facilitate the reactions between NH3 and NOx (Long and Yang, 2002; Delahay et al, 2004; Moreno-Tost et al., 2004; Liu and Teng, 2005), zeolites ZSM-5 and zeolite beta appears to be the most promising. Unlike V2O5/TiO2 SCR catalysts, the zeolite-based catalyst is unable to effectively oxidize NO. Consequently, a metal function, such as iron, cobalt, etc. is added to the acidified zeolite in order to oxidize NO to NO2, which is decomposed according to the following reaction scheme (Long and Yang, 2002):

The rate-limiting step in the reaction sequence is the oxidation of NO to NO2. From equations 1 through 4, ammonia rapidly adsorbs onto the Brønsted acidic sites on the surface of the zeolite to yield adsorbed ammonium ions. NO is oxidized over the metal function (iron oxide) to yield NO2. NO2 interacts with the adsorbed ammonium ions to form an adsorbed ammonium nitrite complex, which decomposes upon reaction with NO to yield N2 and H2O, completing the catalytic cycle. The overall reaction is identical to that of conventional V2O5/TiO2 SCR catalysts.4NH3+4NO+O2→4N2+6H2O  (5)
Tran et al. (U.S. Pat. Nos. 6,689,709 and 6,914,026) report the use of iron exchanged zeolite beta and iron/cerium exchanged zeolite beta in the selective catalytic reduction of NOx using NH3. The catalyst was prepared by ion exchange of zeolite beta using cerium, followed by ion exchange with iron. The catalyst achieved enhanced stability over a material prepared without the addition of cerium. Tran et al. also report the improved stability achieved upon steam treatment at 650° C., 10% H2O for 2 hours. In all cases, catalytic materials lose activity over time.
Absil et al. (U.S. Pat. No. 5,710,085) report improved durability of zeolite beta upon treatment using phosphorous. The resulting material displayed enhanced stability and activity during catalytic cracking studies. Exposure of zeolite to water vapor at an elevated temperature results in the removal of framework aluminum. However hydrothermal treatments under some conditions are known to enhance both the structural stability and acidity of zeolites (Kerr et al., 1970, Breck 1974, Chen and McCullen, 1988; Stevenson et al., 2002).
Despite the above enhancements, prior catalysts do not provide stability and adequate NOx removal efficiency at temperatures above about 550° C. Thus, they cannot efficiently reduce NOx emissions in a direct gas turbine exhaust, such as in simple cycle power plants, without dilution of the exhaust with cooler air. However, dilution of the exhaust significantly reduces the overall gas turbine efficiency, and is not a cost-effective option.